1. Field of the Invention
This invention relates to an automatic actuation system for dual area nozzle assemblies of solid propellant rocket motors for effecting a reduction in the size of the nozzle throat area as required to provide for a long duration sustain phase of motor operation upon completion of an initial boost phase.
2. Description of the Prior Art
Solid propellant rocket motors, in certain applications, require a high thrust boost phase for enabling the rocket to reach the desired altitude and cruising velocity quickly. The velocity and altitude can then be maintained by a longer duration low thrust sustain phase.
The thrust produced by a rocket motor is controlled by the size of the throat area of the rocket motor nozzle and by factors related to the propellant that is used. In rocket motors using a single nozzle and a single combustion chamber, proposals have been made to achieve the difference in thrust level between the boost and sustain phases by appropriate selection of the propellant material and/or grain design. It is not always convenient or possible, however, to achieve the required thrust level ratio in this manner.
Dual thrust rocket motors employing a separate combustion chamber and a separate nozzle for each of the boost and sustain charges are also known. An especially serious objection to this method of providing the required dual thrust is the large weight penalty that is involved.
Dual thrust rocket motors utilizing a single combustion chamber and a nozzle assembly comprising boost and sustain nozzles for varying the size of the nozzle throat area for boost and sustain operation of the motor have also been proposed in the prior art. Rocket motors of this type are disclosed in the following U.S. Pat. Nos.: 3,182,447, H. S. Bell, Jr., 3,011,309, J. M. M. Carter, and 3,230,704, J. J. Lovingham.
In each of the Bell and Carter patents, the combustion chamber of a solid propellant rocket motor terminates at its aft end in a dual area nozzle assembly. The nozzle assembly includes a movable section that is mechanically movable upon completion of the boost phase. That is to say, the movable section is moved aft from a forward position in which it is mechanically restrained during the boost phase. When the movable section is in its forward position, another nozzle section provides the nozzle area required for the boost phase. When moved aft from the forward position, the movable section provides a smaller nozzle area for operation of the motor in a sustain phase.
The Bell patent nozzle assembly includes a booster expansion nozzle and a smaller tubular sustain nozzle that is concentric with the booster nozzle. Axially extending annularly spaced slots are provided in the sustain nozzle through which combustion gases pass during the boost phase, the sustain nozzle then being in a forward position in the combustion chamber. The sustain nozzle is slidably mounted in the combustion chamber and is movable aft to seat in and against the booster nozzle thereby closing and sealing the slots. In one embodiment gas from a high pressure source, not shown or described, is applied, presumably upon external command by means not described, to effect such sliding movement of the sustain nozzle when the booster phase of operations has been concluded. In other embodiments of the Bell patent, upon completion of the booster phase, gas pressure derived by means of conduits that project into a forward part of the solid booster propellant is used to actuate pistons that release detents which hold the sustain nozzle in the forward position. As the solid booster propellant is consumed, the forward ends of the conduits admit pressurizing gas to the pistons. Release of the detents allows the sustain nozzle to slide aft into seating engagement with the boost nozzle under pressure of the gases in the combustion chamber.
The nozzle assembly of the Carter patent includes a nozzle carrier, boost and sustain nozzles that are supported in the carrier in axial alignment with a hydraulic system that holds the sustain nozzle in a forward position, by-pass parts in the nozzle carrier which bypass the sustain nozzle and communicate with the boost nozzle, and a pressure sensitive valve. The pressure sensitive valve is operative at the end of the boost phase, as the combustion chamber as pressure falls, to expel the hydraulic fluid from the system whereby residual motor gas pressure at the end of the boost phase moves the nozzle carrier aft. This seals the by-pass parts in the carrier so that the motor gases can only pass out through the sustain nozzle. Additionally, ball detents holding the boost nozzle in place are released, allowing the boost nozzle to be jettisoned, thereby avoiding an unsuitable expansion ratio while the rocket motor is still at low altitude. In other embodiments in which the hydraulic system is described as preferably dispensed with, the nozzle carrier is released at the end of the boost phase responsively to the pressure drop in the rocket motor as sensed by a pressure sensitive device which sends a firing signal to a series of explosive bolts. This fractures the bolts leaving the nozzle carrier free to move aft under the joint influence of helical coil springs and the residual combustion chamber pressure.
In the Lovingham patent, the area of the thrust nozzle of a liquid propellant rocket motor is adjusted to provide boost and sustain operations by means of a pintle that is adapted to be moved from a boost phase position to a sustain phase position. Pending a command signal, the pintle is retained in a forward position by a ball detent. At the conclusion of the boost phase, a command signal fires a squib cartridge. This causes the release of the ball detent and movement aft of the pintle under the action of oxidizer tank nozzle pressure to reduce the area of the thrust nozzle for sustain operation.
Such prior art rocket motor actuatable nozzle assemblies and actuating structures and controls for providing an initial high thrust boost phase followed by a longer duration low thrust sustained phase are characterized by a number of inherent disadvantages. They are overly complicated and require complicated mechanisms or systems and pressure sensitive devices that add undesirably to the weight, space, resistance and cost of manufacture and assembly of the motor, and moreover, tend to increase the likelihood of operation failure.
Thus, there exists a need and a demand in the art for simplification and improvement in automatic means of actuating a mechanically restrained dual area nozzle assembly of a solid propellant rocket motor for operation in the sustain phase upon the conclusion of the boost phase.